1. Field of the Invention
The present invention relates to electric fish barriers used to govern the motion of fish in water. More particularly, the present invention relates to an electric fish barrier for discouraging fish from entering water intakes disposed at varying depths within a body of water.
2. Description of Related Art
The protection and preservation of natural resources includes the management of fish and game. Fish move about lakes, rivers, streams and reservoirs for a variety of reasons, including migration, spawning, and searching for food. Water intakes divert water for drinking, irrigation, and industrial uses. The introduction of fish into intakes is generally regarded as an unwanted event, and, in some cases, is expressly prohibited by federal government mandates such as the “Endangered Species Act” and the EPA “Clean Water Act.”. Many rivers have hydroelectric, fossil fuel and nuclear power plants with water intakes to the hydroelectric turbines and for cooling. It is desirable to keep the fish out of these intakes and away from dangerous conditions. Many large bodies of water are linked by inland waterways, including natural rivers and man made canals. Some of these bodies of water have diverse fish and wild life that are foreign to each other. Because migration across such natural divides can upset the ecological balance, government mandates often require that construction and use of such waterways incorporate a method or apparatus for controlling ecologically harmful migration through these waterways. As a consequence, all water diversions require governmental licenses and/or permits, and require periodic re-licensing. The water diversions must be upgraded to satisfy any changes in government regulations at the time of re-licensing. For these, and a variety of other economic, commercial, cultural and ecological reasons, it is often necessary to govern the migration and random motion of fish.
As the need for governing the movement and migration of fish has been recognized, means for achieving this goal have also been developed. Electric fish barriers, such as described in U.S. Pat. No. 4,750,451 to Smith, have become a common and useful means for governing the migration and travels of fish in lakes, locks, rivers, dams, fisheries and other restricted or controlled areas.
In electric fish barriers, an electrical irritation or shock is only felt by a fish when there is a voltage differential across the fish thereby driving an electrical current through a fish. Accordingly, the most significant factor in controlling the motion of fish is not the field strength, with respect to ground, where the fish is located, but the voltage gradient where the fish is located. Field voltage gradient is the rate of change in voltage of an electric field per linear measure. Although the instantaneous axis of the linear measurement can be in any direction, the maximum field gradient is measured across a unit length of a one dimensional line oriented perpendicular to the two dimensional surface representing an equipotential voltage plane. The instantaneous voltage differential across unit distance is thus the electric field gradient, or voltage gradient. The higher the voltage gradient, the greater the total voltage drop across a fish, and consequently, the greater the electrical current that will pass through a fish.
Because a gradient times a linear distance equals a voltage potential, it can be understood that the longer a fish, the greater the total voltage drop across the fish. Similarly, because resistance is inversely proportional to the cross sectional area of a resistor, and because a large fish typically has a proportionally larger cross sectional area, the larger the fish, the lower the resistance of the fish. The size of a fish, therefore, affects the electrical current flow through the fish for several reasons as illustrated above.
Although a maximum transfer of energy from water to a fish occurs when the fish's electrical conductivity matches the electrical conductivity of the surrounding water, because a fish has salts and electrolytes within its body necessary to sustain life, a fish's body is normally more conductive than fresh water. As a result, the fish's body acts as a “voltage divider” when swimming through fresh water, and the gradient of an electrical field in the body of a fish will typically be less than the voltage gradient in the same space filled by fresh water. That is, the voltage gradient is altered in a region proximate a fish in the zone of an electric fish barrier. Nevertheless, all other factors remaining equal, the voltage gradient in the body of a fish will be roughly proportional to the voltage gradient in the same region of fresh water when no fish is present. Accordingly, if the voltage gradient in a region of water is doubled, the voltage gradient across the fish (and the electrical current through the fish) will also double. The effectiveness of an electric fish barrier on a particular fish, therefore, depends on the voltage field gradient produced by the electric fish barrier.
If a voltage gradient in a region of water is too weak, the fish will not feel appreciable discomfort, and will travel undaunted by the electric fish barrier. An “annoying region” will cause a fish to turn around and travel the preferred route. Conversely, early experiments have demonstrated that if a moderately annoying region of the electric barrier is too narrow to allow a fish to turn around, and a rapidly swimming fish passes quickly through an “annoying” region and into a painful region, large fish have been observed to react so violently in their attempt to change direction that they have actually snapped their own spine. As a result of these observations, an ideal fish barrier will normally have a wide region with a moderately annoying voltage gradient, increasing at a rate that causes increasing discomfort to fish of various sizes and species, but allowing ample room for a fish experiencing discomfort to turn around before passing completely through the annoying region and into a painful or lethal region. The awareness of the field gradient should, therefore, not be a sudden discovery, but a gradually growing annoyance. Whether a fish barrier is effective, ineffective or harmful is thus a function of the shape of the boundary, the thickness and the intensity of a voltage gradient produced by an electric fish barrier.
The current passing through a fish depends on a variety of factors such as the conductivity of the water at both ends of the fish, the total resistance in a conductive path of water, and the size and species of a fish being repelled, etc. Typically, higher gradients are necessary to control the travel and migration of smaller fish, and lower gradients are effective for larger fish. The effectiveness of a particular strength gradient also depends on the species of fish, and whether the motion of the water reliably flows in a direction to orient the fish along the axis of the strongest gradient, which is perpendicular to the equipotential voltage plane. However, a voltage gradient of one hundred volts per meter has been observed to establish a good base-line voltage gradient for effectively and yet safely deterring average size fish from entering a prohibited area. It is understood that higher and lower voltage gradients may be appropriate according to a variety of factors.
FIG. 1 illustrates a multi-stage fish barrier known in the prior art for regulating the traffic of fish in shallow waterways. According to this example, fish 9 within a waterway 10 seek to migrate up river (against the water flow), and the electric barrier is configured to direct them to an alternative route 11. Five electrodes 13–17 rest on a substrate 12 within riverbed 10. The five electrodes 13–17 separate the stream or river into four separate voltage gradient regions 18–21. The electrodes 13–17 are advantageously formed from elongated members, such as cables or extruded bars. Although copper conducts electricity well, galvanic effects between copper and water can prematurely erode copper cables, requiring frequent replacement. Additionally, in water having a sulfur content, the ionized copper can form copper sulfate compounds in water, which can be poisonous to fish. For these reasons, a ferrous metal is usually preferred for forming the elongated members of the electrodes 13–17, such as steel cables, beams, or railroad track segments. The elongated members 13–17 are oriented perpendicular to the direction of water flow, which, in most confined river areas, also creates a geometrically parallel orientation among the elongated members.
The electrodes 13–17 of FIG. 1 are arranged at one meter intervals, and the voltage levels are controlled such that the relative voltage between two electrodes is continually increasing. Electrode 13 is at a zero or ground potential, and electrode 14 is at a one hundred volt peak potential, so that the peak differential between electrodes 13 and 14 is a one hundred volt differential. Electrode 15 is at a three hundred volts peak potential, so that the peak differential between electrodes 14 and 15 is a two hundred volt differential. Electrode 16 is at a six hundred volts peak potential, so that the peak differential between electrodes 15 and 16 is a three hundred volt differential. Electrode 17 is at a one thousand volts peak potential, so that the peak differential between electrodes 16 and 17 is a four hundred volt differential.
Since the distance between the electrodes 13–17 remains a constant one-meter, the voltage gradient in each region 18–21 is greater than the previous region. In region 18, the gradient is one hundred volts per meter. In region 19, the gradient is two hundred volts per meter. In region 20, the gradient is three hundred volts per meter.
In region 21, the gradient is four hundred volts per meter. As a fish advances into a progressively higher voltage gradient, the electrical current passing through that fish increases proportionally. Through the multi-stage barrier of FIG. 1, fish of a size or species that are not annoyed by a lower voltage gradient will be progressively exposed to higher voltage gradients, eventually forcing all migrating fish to turn around and select the alternative path 11 in their upstream travels. Although the multi-step barrier of FIG. 1 can be effective in a shallow stream, the incremental regulation of voltage gradients is not reliably formed by single-step or multi-step designs of the prior art in deeper water applications.
FIG. 2 is a prior art cross sectional view of a stream or river nine meters deep, illustrating the equi-gradient field lines of an electric field produced by two elongated members 30, 31 on a riverbed. The direction of river flow is along the w-axis. The elongated members 30, 31 are separated by fourteen meters in the direction of river flow, and disposed at the bottom of a river 32, perpendicular to the direction of flow. The conductivity of the river water is 500μ Siemens. A one kilovolt differential is generated between the two elongated members 30, 31.
As discussed above, the basic operational parameter of an electric fish barrier is the voltage gradient of an electric field, and a gradient of 100 volts per meter is a common benchmark for an operational system. If the field gradient between the two conductors 30, 31 were completely linear, one thousand volts over a fourteen meter range would produce a continuous gradient of seventy-one volts per meter. As the field gradient patterns of FIG. 2 indicate, however, the field gradient is not uniform between the two conductors 30, 31. A field gradient of sufficient strength must extend all the way to the surface to prevent passage of fish past the barrier. Because fish can travel on the surface where the gradient is weakest, the strongest gradient value to extend all the way to the surface is an important value for profiling the efficacy of a fish barrier. The strongest voltage gradient extending to the river surface in FIG. 2 was measured at 25 volts per meter. On the bottom of the riverbed, near the conductive elongated members 30, 31 viewed end-wise, the higher field gradients more closely resemble concentric cylinders formed around the respective elongated conductive members 30, 31. As one approaches the conductive members 30, 31, the path leading to a conductive member 30, 31 is distinguished by a voltage potential that changes rapidly over distance, which equates to a high voltage gradient.
Because effective blocking of fish from migrating up or downstream would require a minimum gradient of 100 volts per meter everywhere in a cross-sectional plane to the direction of flow of the river, calculations were performed normalizing the surface gradient at one hundred volts per meter according to the prior art design of FIG. 2. At this normalized value, the calculations disclose that a peak voltage difference of 4.032 kilovolts between the elongated members 30, 31 would be required to produce a surface gradient of one hundred volts per meter.
At the level of 4.032 kilovolts potential between the elongated members 30, 31, the electrical current produced by the normalized electric field pattern in a river nine meters deep and one meter wide would be 52.5 amps at a conductivity of 500μ Siemens.
Although fish barriers can be formed from any kind of signal, past discoveries have indicated that A.C. barriers can have harmful physiological effects on fish, and that steady state D.C. barriers can be counter-productive in preventing downriver migration, progressively tetanizing a fish as it enters the steady state D.C. electric field, thereby diminishing its ability to resist the flow of water, and inexorably carrying the fish downriver. D.C. pulse trains ranging from two to ten pulses per second, with pulse width ranging from 400 microseconds to 40 milliseconds have been found to be safe and effective in many applications. When a D.C. pulse at a 2% duty cycle (ratio of on-time to the total time period of a pulse) is incorporated at the voltage of 4.032 kilovolts in the prior art design of FIG. 2, the power consumption is 4,234 kilowatts for a river nine meters deep and 1 meter wide at 500μ Siemens. The 4.032 kilovolt level and the 2% duty cycle would remain unchanged for any width river, provided the elongated members 30, 31 extend across the entire riverbed. The above values for electrical current and power can be adjusted linearly for any width river. For example, a ten meter wide river would require a peak current of 525 amps at a power consumption of 42.34 kilowatts.
An additional problem with previous technology is that the most intense voltage gradient is proximate the floor of the river, reservoir or river. Many reservoirs, however, are quite deep, and water intakes in dams and irrigations reservoirs can be located significant distances from the floor of a reservoir. As noted above, the gradient diminishes the further one travels from the floor of the river or reservoir. In a reservoir wherein a water inlet is thirty or forty feet above the floor, extending a sufficiently strong voltage gradient from the floor of the reservoir to the front of the water inlet could consume enormous amounts of energy, and could potentially electrify the entire reservoir to dangerous levels.
There is, therefore, a need for an electric fish barrier that can form an effective voltage gradient in front of a water inlet in a reservoir. There is further a need for an electric fish barrier that can form an effective voltage gradient at a distance from the bottom of a reservoir without using enormous amounts of energy. There is further a need for an electric fish barrier that can focus a voltage gradient in front of a water inlet in a reservoir without unduly extending a strong voltage gradient to other regions of a reservoir.